EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 73 



agreed that it is. But the long and careful studies of Hadzi (1953) have 

 thrown doubt on its correctness. Despite the striking parallels noted, we 

 may not have here a case of recapitulation at all. The metazoa may have 

 evolved by an entirely different process. Hadzi presents reasons for con- 

 cluding that protozoa of the type known as Infusoria (cilia-covered or- 

 ganisms like Paramecium) gave rise to very simple members of Phylum 

 Platyhelminthes, the Acoela of Class Turbellaria. These minute worms 

 have no digestive cavities and are provided with ciha. Some of the In- 

 fusoria have many nuclei in the single large cell. If a bit of cytoplasm 

 around each nucleus became enclosed by a cell membrane a many-celled 

 organism would result. Hadzi visualizes the Acoela as arising from the 

 Infursoria by such a process of cellularization (see de Beer, 1954). Ac- 

 cording to this view coelenterates are somewhat simplified descendants of 

 flatworms (Platyhelminthes), not their ancestors. The simplification has 

 been connected with adoption by coelenterates of an attached or sessile 

 mode of existence. 



Since there are no known fossils of the first metazoan animals, we have 

 no direct evidence as to whether metazoa arose by aggregation of cells 

 as seen today in colonial protozoans (Haeckel's view; reaffirmed by Mar- 

 cus, 1958) or by cellularization of cilia-bearing protozoans (infusorians) 

 as postulated by Hadzi (1953). The embryonic development of higher 

 animals seems to indicate that Haeckel was correct; yet in the absence of 

 other evidence it is dangerous to conclude that the early stages of em- 

 bryonic development necessarily represent recapitulation of what hap- 

 pened when the protozoa gave rise to the metazoa many millions of years 



ago. 



Recapitulation Aids in Classification 



Recapitulation is sometimes of aid in informing us of the relationships of 

 animals when the adult structure leaves us in doubt. A case in point is pre- 

 sented by the tunicates. Members of the genus Molgula (Fig. 4.16) are 

 small, soft-bodied, "spineless" creatures living in the sea. Their outer sur- 

 face is a tunic (hence the name "tunicate") of cellulose. They live at- 

 tached to rocks, the pilings of wharves, and the like. The tunic is pierced 

 by two openings corresponding to the incurrent siphon ("mouth" in Fig. 

 4.16) and the excurrent siphon ("opening of atrium") of a clam or oyster. 

 Sea water is sucked in through the incurrent siphon, is passed into a 

 pharynx the walls of which are constructed to strain out tiny animals and 

 plants suitable for food, and, after passing through the "pharyngeal gill 



